A variety of electrochemical cells falls within a category of cells often referred to as solid polymer electrolyte (“SPE”) cells. An SPE cell typically employs a membrane of a cation exchange polymer that serves as a physical separator between the anode and cathode while also serving as an electrolyte. SPE cells can be operated as electrolytic cells for the production of electrochemical products or they may be operated as fuel cells.
Fuel cells are electrochemical cells that convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. A broad range of reactants can be used in fuel cells and such reactants may be delivered in gaseous or liquid streams. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen containing reformate stream, or an aqueous alcohol, for example methanol in a direct methanol fuel cell (DMFC). The oxidant may, for example, be substantially pure oxygen or a dilute oxygen stream such as air.
In SPE fuel cells, the solid polymer electrolyte membrane is typically perfluorinated sulfonic acid polymer membrane in acid form. Such fuel cells are often referred to as proton exchange membrane (“PEM”) fuel cells. The membrane is disposed between and in contact with the anode and the cathode. Electrocatalysts in the anode and the cathode typically induce the desired electrochemical reactions and may be, for example, a metal black, an alloy or a metal catalyst supported on a substrate, e.g., platinum on carbon. SPE fuel cells typically also comprise a porous, electrically conductive sheet material that is in electrical contact with each of the electrodes, and permit diffusion of the reactants to the electrodes. In fuel cells that employ gaseous reactants, this porous, conductive sheet material is sometimes referred to as a gas diffusion backing and is suitably provided by a carbon fiber paper or carbon cloth. An assembly including the membrane, anode and cathode, and gas diffusion backings for each electrode, is sometimes referred to as a membrane electrode assembly (“MEA”). Bipolar plates, made of a conductive material and providing flow fields for the reactants, are placed between a number of adjacent MEAs. A number of MEAs and bipolar plates are assembled in this manner to provide a fuel cell stack.
In fabricating unitized MEAs, multilayer MEAs may be prepared and then cut to the required size. In this process, stray fibers from the electrically conductive electrode material may bridge across the thin membrane, interconnecting the electrodes that could result in short-circuiting in an operating fuel cell.
Conventional MEAs have also been made that incorporate a membrane having a larger surface area than the electrode layers, with at least a small portion of the membrane extending laterally beyond the edge of the electrode layers. This prevents short-circuiting between the electrodes around the edge of the membrane. However, in MEA fabrication, this poses a problem because the membrane and the electrode layers have to be cut separately resulting in a slow-down of the manufacturing process and loss of productivity.
Another route to fabricating a unitized membrane electrode assembly described in U.S. Pat. No. 6,057,054, in particular, relies on the use of curable or thermosetting sealing materials. Thermosetting materials are materials that, once heated, react irreversibly so that subsequent applications of heat and pressure do not cause them to soften and flow. In this case, a rejected or scrapped piece cannot be ground up and remolded.
The chemical nature of thermosetting materials may lead to undesirable process and product attributes in the fuel cell application. Such materials often require a relatively long time period for chemical reaction to solidify them; they are commonly used for low-volume parts manufacture, where fast cycle times are not important. Reacting components in thermosetting materials may generate undesirable gaseous emissions, requiring forced ventilation of the production area, raising environmental issues, and forming bubbles in the solidified part. The chemical nature and reactivity of residual low-molecular-weight components in thermosetting materials present the possibility of contamination of the membrane or catalyst in a fuel cell. Further, unreacted chemical functional groups in the thermosetting materials provide likely sites for corrosion or other attack by the acidic species in the fuel cell environment.
A further deficiency of thermosetting materials, as implied in the description above, is the inability to repair or recycle a defective part. Such a problem can significantly hurt the economics of manufacture, as the yield loss of the valuable MEA contributes strongly to manufacturing cost.
Physical properties of thermosetting materials are also often insufficient for durable fuel cell operation. Many lack mechanical toughness, and low-modulus forms such as natural rubber require very precise control of stack pressure to assure the desired MEA thickness when assembled.
Finally, the processing of curable resins introduces difficulties in fabrication of the unitized MEA. The relatively low viscosity of the uncured material makes it difficult to control and limit its flow into the porous diffusion backings. Excessive flow can seal off part of the expensive catalyst layers, rendering it inactive. In some cases, a raised dike as described in U.S. Pat. No. 6,057,054 is necessary to mitigate this problem. Quite importantly, thermosetting materials generally do not solidify during the flow process but remain fluid throughout until the flow is stopped and the cure progresses significantly. This causes the flow to be relatively uniform, leaving the MEA components in essentially the same positions they were in before the introduction of the resin. When a short circuit is present before introduction of the resin, as from portions of the electrode layers that inadvertently straddle the membrane, it remains sealed in place after fabrication.
A need exists for polymers useful sealant materials for MEAs that do not have the problems associated with thermosetting resins described above.